The magnetic characterization of T and R states of paramagnetic forms of Hemoglobins A and Kansas has been studied. Analogous structures have been demonstrated for bacterial tryptophan oxygenase in the absence or presence of substrate, respectively. Model compound studies of heme NO complexes and analyses of the EPR of low spin ferric Hemoglobins A and Kansas and bacterial tryptophan oxygenase in the low affinity form leads us to conclude that a major aspect of the Bohr effect as well as other linked functions in hemoglobin, are associated with the state of protonation of the proximal imidazole bound to the heme. The linear electric field effect in EPR is a new biophysical tool used to calculate odd contribution to crystal fields of low spin ferric heme compounds. It is concluded in these studies that the major bonding of axial ligands to heme iron is through pi-delta overlap, with the distribution of heme energy levels dependent upon the nature of the axial ligands.